Model (or Object) space refers to the actual coordinates in the geometry (like
terrain tiles, an airplane model, etc). In OSG, model coordinates might be absolute
or they might be transformed with an OSG Transform.

We will often refer to two types of Model coordinates: world and local.

World coordinates are expressed in absolute terms; they are not transformed.
Local coordinates have been transformed to make them relative to some reference
point (in world coordinates).

Why use local coordinates? Because OpenGL hardware can only handle 32-bit values for
vertex locations. But in a system like osgEarth, we need to represent locations with
large values and we cannot do that without exceeding the limits of 32-bit precision.
The solution is to use local coordinates. OSG uses a double-precision MatrixTransform
to create a local origin (0,0,0), and then we can express our data relative to that.

Clip coordinate are what you get after applying the view volume (also know as the
camera frustum). The frustum defines the limits of what you can see from the eyepoint.
The resulting coordinates are in this system: